46 



GENERAL PAKT. 



sheaths, which are prolonged over their processes and so over the 



nerve fibres. Very generally several ganglion cells are enclosed 



in a common sheath. 



Nerve fibres are either centrifugal, i.e., they carry nervous impulses 



from the central organ to the peripheral organs (motor, secretory 



nerves) ; or they are centripe- 

 tal, i.e., they carry them from 

 the periphery to the central 

 organs (sensory nerves). They 

 are prolongations of ganglion 

 cells, and, like them, are fre- 



FIG. 38. Nerve fibres (partly after M. 

 Schultze). a, non-medullated sympa- 

 thetic fibre, b, medullated fibres, one 

 of them with commencing coagulation 

 of the axis cylinder, c, medullated 

 nerve fibre with the sheath of 

 Schwann. 



FIG. 37. a bipolar ganglion cell, b, nerve cell, 

 from the human spinal cord (anterior cornu), 

 (after Gerlach). P, pigment body. 



quently enclosed in a nucleated sheath. 

 The larger and smaller nerves are 

 composed of a number of such fibres 

 bound together. According to the 

 minute structure of the nervous sub- 

 stance we distinguish two kinds of 

 nerve fibres (1) the so-called medullated nerves, with a double 

 contour; (2) the non-medullated or naked axis cylinders (fig. 

 38, a, b, c). 



The former are distinguished by the fact that, on the death of 

 the nerve and as the result of coagulation, a strongly refractile 

 fatty substance which forms a sheath for the nerve fibre comes into 

 view. This sheath is known as the medullary sheath, and the 

 central fibre as the axis cylinder. The medullary sheath disappears 

 near the ganglion cell, the axis cylinder only entering the protoplasm 



